docs: def is_metric_cls(cls):
      """A metric class is defined as follows:
  
          - It inherits DiffParserBase
          - It is not DiffParserBase
          - It does not have __metric__ = False
      """
      return (
          cls is not DiffParserBase and
          cls.__dict__.get('__metric__', True) and
          issubclass(cls, DiffParserBase)
      )